1. Field of the Invention
The present, invention relates to an ultrasonic motor which generates a driving force using elastic vibration of a piezoelectric element and, more specifically, to the structure of a mover and a vibrator in the ultrasonic motor.
2. Description of the Prior Art
A known example of an ultrasonic motor's vibrator is the one disclosed by U.S. Pat. No. 4,580,073. FIG. 1 shows the structure of this conventional ultrasonic motor.
In FIG. 1, a reference numeral 2 indicates a ring vibrator, a reference numeral 3 indicates a mover (or movable member) and a reference numeral 2a represents projections. The projections extend in a direction transverse to the direction of advance of the travelling bending elastic wave and are disposed at equal angular spaces about the entire circle of the ring vibrator 2. A reference numeral 2b designates a base ring portion which is on the opposite side of the projections 2a and integral with the projections 2a. The base ring portion 2b has a piezoelectric element 4 (see FIG. 2) which has a polarized structure as shown in FIG. 2.
FIG. 2 illustrates an example of the polarized structure of the piezoelectric element 4 which oscillates nine elastic waves along the circle of the vibrator 2. Each of portions A and B contains a group of electrodes, each constituting a small polarized domain corresponding to a half-wave length of the elastic wave. Portion C is an electrode corresponding to a three-quarter wave length. Portion D is an electrode corresponding to a one-quarter wave length. Electrodes C and D are disposed there to spatially provide a phase difference of one-quarter wave (equal to 90.degree.) length between two groups of electrodes A and B. In two groups of electrodes A and B, any two adjacent small domains are polarized in the direction of the axis of the ring vibrator 2 so that they have opposite polarities with respect to one another. The piezoelectric element 4 is rigidly attached, at the opposite side of the piezoelectric element surface shown in FIG. 2, to the base ring portion 2b. Each electrode is flush with the surface of the piezoelectric element. When the ultrasonic motor is in operation, the electrode group A is short-circuited as hatched in FIG. 2. The electrode group B is also short-circuited as hatched in FIG. 2.
Now let's assume that voltages V.sub.1 and V.sub.2 expressed as below are applied to the electrode groups A and B of the piezoelectric element 4, respectively. EQU V.sub.1 =V.sub.0 .times.sin (.omega. t) (1) EQU V.sub.2 =V.sub.0 .times.cos (.omega. t) (2)
where V.sub.0 is the amplitude of the applied voltage, .omega. is an angular frequency, and t is time.
A bending travelling wave as expressed by the following equation is generated on the ring vibrator 2, and advances along the circle of the ring vibrator 2. ##EQU1## where .xi..sub.0 is the amplitude of the bending vibration, .xi. is an instantaneous value of the bending vibration, k is the number of waves (2.pi./.lambda.), .lambda. is the wavelength, and x is a position of interest.
A point P of projections 2a is, accordingly, in motion in an elliptical orbit. The mover 3 in contact with the point P is driven, by means of frictional force, in the opposite direction X' of the direction X in which the travelling wave advances on the ring vibrator 2.
For the purpose of increasing the rotational speed at the point P in the ultrasonic motor, the above-mentioned vibrator 2 is provided with projections 2a which involves a difficult machining process. The provision of the projections 2a causes attainment of the above goal by increasing transverse displacement.
One example of the structure of a mover of a conventional ultrasonic motor is disclosed by Japanese Laid Open Publication No. 63-174581. Discussed in that disclosure are relative relationships of various shapes of the mover, which has a structure of a cantilever flange made of aluminum or aluminum alloy. It is also disclosed that the generation of abnormal mechanical noise is prevented by optimizing rigidity of the flange.
An important factor in the structure of the conventional ultrasonic motor is the accuracy of the flatners between the projections 2a extending from the base ring portion 2b and of the mover 3. Both mechanical strength variations in attachment portions where the projections 2a join the base ring portion 2b and variations in machining accuracy of the height of the projections 2a affect a resonant frequency at which both electrode groups A and B vibrate, thereby varying the vibration displacement and consequently reducing the efficiency of the ultrasonic motor. To overcome the above problem, the flatness accuracy needs improving. Improvement in the flatness accuracy involves longer machining times and very rigorous machining accuracy requirements. These requirements in the production stage are incompatible with low production costs and a mass production process. The resonant frequency of the projections (a function of the width, depth, and height of each projection) should not ,meet with the driving frequency of the vibrator; actual the machining process, by nature, sets a design limit of the minimum space allowed between adjacent projections; and sufficient mechanical strength of the projections is required for the output to be efficiently picked up. All these requirements set a limit an the number of projections allowed. On one hand, the use of projections contributes to the increase of the rotational speed of the motor. On the other hand, however, the structure of the projections causes amplitude variations in the vibration, because of changes in the resonance characteristics directly resulting from a periodic change in the rigidity of the vibrator. The unavoidable design limit imposed on the number of projections adversely affects motor characteristics, for example, causing cogging or other problems, particularly when the motor is operated at a low speed operation.
To sum up, in the above ultrasonic motor, cracks tend to develop at the root of the projection, and parts of the ring vibrator to which the piezoelectric element is fitted are limited because of a periodic change in a circumferential direction of rigidity in the ring vibrator, thus assembly work for the piezoelectric element is troublesome.
When the mover is of the cantilever flange structure, the problem is as follows:
With the vibrator pressed into contact with the cantilever flange mover, the cantilever flange mover is deflected and distorted with respect to its fixed end, and fails to be uniformly in contact with the vibrator. Therefore, reliable a contact state between the vibrator and the mover is not assured. To improve on this problem, the contact surface is very narrow. A narrow contact surface, however, forces more contact pressure, leading to wear of each contact surface of both the vibrator and the mover, and even leading to seizure of both. To avoid such problems, the contact surface needs to be coated with a hard material, such as an oxide compound. Such an additional process is not acceptable from the standpoint of cost and mass production. If such hardening process is employed, a motor service life is determined by the capability of the hard material to remain adhered to the vibrator. That adherence capability is definitely unpredictable and unstable. Furthermore, the flange type mover is complex in its structure, and presents various difficulties in the machining process. For example, when a small pressure of several Newtons is applied or the motor diameter is miniaturized, the thickness of the flange mover is extremely thinned. Such a technique is not applicable in a wide range of applications, because prior-art structures have numerous limitations on the machining process, the shape of the ultrasonic motor and the pressure ranges applied.